Fixes <rdar://23122310> Runtime dynamic casts...
This makes runtime dynamic casts consistent with language rules, and
consequently makes specialization of generic code consistent with an
equivalent nongeneric implementation.
The runtime now supports casts from Optional<T> to U. Naturally the
cast fails on nil source, but otherwise succeeds if T is convertible to
U.
When casting T to Optional<U> the runtime succeeds whenever T is
convertible to U and simply wraps the result in an Optional.
To greatly simplify the runtime, I am assuming that
target-type-specific runtime cast entry points
(e.g. swift_dynamicCastClass) are never invoked with an optional
source. This assumption is valid for the following reasons. At the
language level optionals must be unwrapped before downcasting (via
as[?!]), so we only need to worry about SIL and IR lowering. This
implementation assumes (with asserts) that:
- SIL promotion from an address cast to a value casts should only happen
when the source is nonoptional. Handling optional unwrapping in SIL
would be too complicated because we need to check for Optional's own
conformances. (I added a test case to ensure this promotion does not
happen). This is not an issue for unchecked_ref_cast, which
implicitly unwraps optionals, so we can promote those!
- IRGen lowers unchecked_ref_cast (Builtin.castReference) directly to
a bitcast (will be caught by asserts).
- IRGen continues to emit the generic dynamicCast entry point for
address-casts (will be caught by asserts).
There was previously no way to detect a type that is nominally
Optional at runtime. The standard library, namely OutputStream, needs
to handle Optionals specially in order to cirumvent conversion to the
Optional's wrapped type. This should be done with conditional
conformance, but until that feature is available, Builtin.isOptional
will serve as a useful crutch.
This reverts commit 0d001480a9.
The commit that changed the iteration style from c-based loops into for-each
loops caused major regressions in our string benchmarks. Arnold believes that we
are making a different inlining decision in the for-each loops. We should
reapply this patch after we fix the optimizer
The regression was detected in rdar://23776732.
Also remove dependent calls to hoistableNativeTypeCheck() and checkSubscript()
calls. To find the guarding checkSubscript() call we introduce a return value to
checkSubscript that is used by the _getElement() call so that we can just follow
the use-def chain to find the dependence.
